This invention relates to relay testers, and more particularly to a relay tester in which the static characteristic measurement of a relay can be achieved readily by digital control without using measuring instruments such as a frequency meter, a phase meter, a voltmeter or an ammeter.
A typical example of a conventional relay tester of this type is a slidac type relay tester which comprises a voltage transformer, a current transformer and a slidac and uses a variable frequency generator of several kilo-volt-amperes (KVA) as a test power source. A second example is an analog type relay tester comprising a low frequency oscillator having a small capacity, an analog phase varying unit, an analog type gain varying unit and a power amplifier.
The analog type relay tester is as shown in FIG. 1, and comprises a low frequency oscillator 1 having an oscillator circuit and a frequency adjusting variable resistor VR.sub.1 ; phase varying units 2 each having a phase shift circuit made up of an operational amplifier and a phase adjusting variable resistor VR.sub.2 ; gain varying units 3 each having a gain varying circuit made up of an operational amplifier and a gain adjusting variable resistor VR.sub.3 ; power amplifiers 4; output conversion circuits 5 which are made up of a tapped voltage transformer and a tapped current transformer, respectively; a frequency meter 6; a voltmeter 7; an ammeter 8; a phase meter 9; and output terminals 10 and 11. In FIG. 1, reference numeral 12 designates a device under test, namely, a relay.
The operation of the analog type relay tester is as follows: The low frequency oscillator 1 generates an analog signal of several volts having a desired frequency. In one of the phase varying units 2, an analog signal of several volts having a desired phase is obtained by using the variable resistor VR.sub.2 with the output signal of the oscillator 1 as a reference. In one of the varying units 3, coupled to the one phase varying unit 2, the output signal of the latter 2 is received, and the gain is adjusted to a desired value by using the variable resistor VR.sub.3, so that an analog signal of several volts is outputted. The analog signal thus outputted is too small to drive the device 12 under test. Therefore, the analog signal is applied through a power amplifier 4 to an output conversion circuit 5. The output conversion circuit 5 has taps for determining the dynamic range of its output voltage; i.e., it is so designed as to be able to select a desired dynamic range. The output voltage of the output conversion circuit 5 is applied, as a voltage source, to the output terminal 10, the frequency meter 6, the voltmeter 7 and the phase meter 9, and to the device 12 under test.
On the other hand, a current source is provided for the device 12 under test through the output terminal 11 of the lower circuits 2, 3, 4 and 5 which are substantially similar in arrangement to the above-described circuits.
As is apparent from the above description, in order to give a static characteristic test to a relay, it is necessary for the operator to manually set the frequency, phase, voltage value and current value with the variable resistors according to the test items of the relay and to monitor indications on the meters to ensure that the proper signal levels are applied. In other words, since the conventional test is arranged as described above, the operator must manually either coarsely or finely adjust the frequency, phase, voltage value and current value to be applied to the relay with the variable resistors while monitoring the meters, which meters must be very accurate to permit accurate control of the applied values. Such a test method, which depends on the operator's intuition and visual detection, not only contributes to the non-uniformity of products, but also hinders working efficiency.